1. Field of the Invention
The present invention relates to programming non-volatile memory with reduced program disturb.
2. Description of the Related Art
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to the traditional, full-featured EEPROM.
Both the traditional EEPROM and the flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory element can be programmed/erased between two states, e.g., an erased state and a programmed state. Such a flash memory device is sometimes referred to as a binary flash memory device because each memory element can store one bit of data.
A multi-state (also called multi-level) flash memory device is implemented by identifying multiple distinct allowed/valid programmed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits encoded in the memory device. For example, each memory element can store two bits of data when the element can be placed in one of four discrete charge bands corresponding to four distinct threshold voltage ranges.
Typically, a program voltage Vpgm applied to the control gate during a program operation is applied as a series of pulses that increase in magnitude over time. In one possible approach, the magnitude of the pulses is increased with each successive pulse by a predetermined step size, e.g., 0.2-0.4 V. Vpgm can be applied to the control gates of flash memory elements. In the periods between the program pulses, verify operations are carried out. That is, the programming level of each element of a group of elements being programmed in parallel is read between successive programming pulses to determine whether it is equal to or greater than a verify level to which the element is being programmed. For arrays of multi-state flash memory elements, a verification step may be performed for each state of an element to determine whether the element has reached its data-associated verify level. For example, a multi-state memory element capable of storing data in four states may need to perform verify operations for three compare points.
Moreover, when programming an EEPROM or flash memory device, such as a NAND flash memory device in a NAND string, typically Vpgm is applied to the control gate and the bit line is grounded, causing electrons from the channel of a cell or memory element, e.g., storage element, to be injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory element is raised so that the memory element is considered to be in a programmed state. More information about such programming can be found in U.S. Pat. No. 6,859,397, titled “Source Side Self Boosting Technique For Non-Volatile Memory,” and in U.S. Patent Application Publication 2005/0024939, titled “Detecting Over Programmed Memory,” published Feb. 3, 2005; both of which are incorporated herein by reference in their entirety.
During programming of a selected memory element, neighboring memory elements may be inadvertently programmed in a process referred to as program disturb. For example, a memory element that is not meant to be programmed, but which is on the same word line as a memory element selected for programming, may become inadvertently programmed when Vpgm is applied to the word line. Several techniques can be employed to prevent program disturb. For example, with self boosting, the channels associated with the unselected bit lines are electrically isolated and a pass voltage (e.g., 10 V) is applied to the word lines associated with the unselected memory elements during programming. The unselected word lines couple to the channels associated with the unselected bit lines, causing a voltage (e.g., 8 V) to exist in the channel of the unselected bit lines, which tends to reduce program disturb. Thus, self boosting causes a voltage boost to exist in the channel which tends to lower the voltage across the tunnel oxide and hence reduce program disturb. Furthermore, Local Self Boosting (LSB) and Erased Area Self Boosting (EASB) attempt to reduce program disturb by isolating the channel of previously programmed elements from the channel of the element being inhibited.
However, with ever decreasing channel lengths in memory elements, e.g., 90 nm or less, the ability of conventional channel boosting techniques to reduce program disturb is compromised. In particular, the channel length of the memory elements can become too short to sufficiently isolate the two separately boosted channel areas at the drain and source sides of the selected word line. As a result, the boosted channel voltage can be lowered, thereby worsening program disturb. Additionally, band-to-band tunneling or gate induced drain leakage (GIDL) related breakdown can occur near the drain of the grounded word line. Due to this breakdown, the boosted channel can be discharged, causing program disturb, and/or hot carriers may be generated that are injected in the tunnel oxides or into the floating gates of the memory cells. An improved program disturb reduction method is needed which addresses the above and other issues.